1. Field of the Invention
The present invention relates to a lithographic apparatus comprising a movable part and a controller to control a position quantity of the movable part. Further, the present invention relates to a method to control a position quantity of a movable part of a lithographic apparatus.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g., including part of, one, or several dies) on a substrate (e.g., a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Usually, a lithographic apparatus comprises a plurality of movable parts such as a substrate table or substrate stage to hold a substrate, a mask table to hold a patterning device, an optical element of a projection system of the lithographic apparatus, a substrate handler to handle the substrate, etc. Usually, such a movable part is to be positioned with a high degree of accuracy. Therefore, a controller is provided to control a position quantity of the movable part, such as a position, a speed, acceleration, a jerk, etc. of the movable parts. The controller may comprise any type of controller, comprising, e.g., a feed back, a feed forward or a combination of a feed back and a feed forward. The controller may, e.g., make use of sensors, which provide a signal representative of the position quantity or of any other quantity, e.g., a quantity derived from the position quantity. Examples of such sensors are position sensors, speed sensors, acceleration sensors, force sensors, vibration sensors, etc. Further, other information may be provided to the control loop, such as a setpoint signal, a feed forward signal, a feed forward error correction signal, etc. A transfer characteristic of the controller, e.g., a gain, a frequency characteristic, or any other transfer characteristic is according to the state of the art commonly optimized to fulfil two goals: at first, a fast response of the controller is to be achieved, to, e.g., shorten a settling time of the movable part. Secondly, a disturbance rejection by the controller is to be maximized, hence reducing an effect of disturbances (such as noise, etc.) on the position quantity. Commonly, a compromise has to be searched, as optimizing the controller for a high disturbance rejection would deteriorate a speed thereof, while on the other hand in case that the controller would be dimensioned such as to obtain a high speed, disturbance rejection would be marginal.